Lecture Exercises

This page contains five exercises that reinforce the concepts introduced in Lecture 10: ROS 2 (Introduction).

Each exercise is designed to be small but concrete, so that you can practice both the conceptual and hands-on aspects of ROS 2.

Exercise 1 – Conceptual ROS 2 Graph (Nodes, Topics, Messages)

Goal

Reinforce your understanding of the ROS 2 computation graph: nodes, topics, and messages.

Task

  1. Consider a simple mobile robot with the following functionality:

    • A node that reads wheel encoders and publishes odometry.

    • A node that receives velocity commands and sends low-level motor commands.

    • A node that runs a teleoperation interface (keyboard or joystick).

    • A node that estimates the robot’s pose from odometry and IMU.

  2. On paper or using a diagramming tool, draw the ROS 2 graph for this system:

    • Show each node as a box.

    • Show each topic as a named arrow between nodes.

    • Label message types when possible (for example, nav_msgs/msg/Odometry, geometry_msgs/msg/Twist).

  3. Identify for each topic:

    • Which node(s) publish.

    • Which node(s) subscribe.

    • Whether the data flow is “sensor-like” (one-way streaming) or “command-like” (actuation).

Deliverables

  • A one-page diagram of your ROS 2 graph (PDF, PNG, or clear photo).

  • A short paragraph (4–6 sentences) explaining why ROS 2’s modular design is helpful for this system (for example, easier debugging, swapping nodes, simulation vs. real robot).

Exercise 2 – Create and Build a ROS 2 Workspace

Goal

Create a ROS 2 workspace from scratch, add a minimal C++ package, and build it with colcon.

Task

  1. Create a workspace:

    mkdir -p ~/enpm702_ws/src
cd ~/enpm702_ws

  2. Source your ROS 2 installation (adjust to your distribution):

    source /opt/ros/humble/setup.bash

  3. In the src directory, create a new package named enpm702_l10_demo:

    cd ~/enpm702_ws/src
ros2 pkg create enpm702_l10_demo --build-type ament_cmake --dependencies rclcpp

  4. Return to the workspace root and build:

    cd ~/enpm702_ws
colcon build

  5. After a successful build, source the workspace:

    source install/setup.bash

  6. Use ros2 pkg list to confirm that your package is visible.

Deliverables

  • A short text file (for example, exercise2_summary.txt) that includes:

    • The exact commands you used.

    • The output of:

      ros2 pkg list | grep enpm702_l10_demo

    • A brief note (2–3 sentences) describing what “overlaying” the workspace means and why you must source install/setup.bash.

Exercise 3 – Minimal C++ Node with Logging

Goal

Write, build, and run a minimal C++ ROS 2 node using rclcpp and verify that spinning works.

Task

  1. In the package enpm702_l10_demo, create a source file src/minimal_node.cpp that:

    • Initializes ROS 2.

    • Creates a node named "minimal_node".

    • Logs a message at startup.

    • Spins until you stop it.

    Example structure:

    #include "rclcpp/rclcpp.hpp"

int main(int argc, char * argv[])
{
  rclcpp::init(argc, argv);

  auto node = std::make_shared<rclcpp::Node>("minimal_node");
  RCLCPP_INFO(node->get_logger(), "Minimal node has started.");

  rclcpp::spin(node);
  rclcpp::shutdown();
  return 0;
}

  2. Update CMakeLists.txt to:

    • Add an executable target called minimal_node.

    • Link it with rclcpp.

    • Install the target to lib/${PROJECT_NAME}.

  3. Rebuild your workspace and source it.

  4. Run the node:

    ros2 run enpm702_l10_demo minimal_node

  5. In a separate terminal, list running nodes:

    ros2 node list

    Confirm that /minimal_node appears.

Deliverables

  • The minimal_node.cpp file.

  • The relevant snippet from CMakeLists.txt that shows how you added and installed the executable.

  • A brief note (2–3 sentences) explaining what happens if you omit rclcpp::spin(node); from main.

Exercise 4 – String Publisher and Subscriber Pair

Goal

Create a pair of nodes: a publisher that periodically publishes a string message, and a subscriber that prints whatever it receives.

Task

  1. In enpm702_l10_demo, add two C++ source files:

    • src/minimal_publisher.cpp

    • src/minimal_subscriber.cpp

  2. Implement the publisher:

    • Node name: "minimal_publisher".

    • Topic name: "chatter".

    • Message type: std_msgs/msg/String.

    • Use a timer to publish a message every 0.5 seconds, for example "Hello from ENPM702 L10".

  3. Implement the subscriber:

    • Node name: "minimal_subscriber".

    • Topic name: "chatter".

    • Message type: std_msgs/msg/String.

    • In the callback, print the received string using RCLCPP_INFO.

  4. Update CMakeLists.txt to:

    • Add executables for both nodes.

    • Link them with rclcpp and std_msgs.

    • Install the executables.

  5. Rebuild the workspace and source it.

  6. Run the nodes in two terminals:

    # Terminal 1
ros2 run enpm702_l10_demo minimal_publisher

# Terminal 2
ros2 run enpm702_l10_demo minimal_subscriber

  7. Use ROS 2 CLI tools to inspect the system:

    • ros2 node list

    • ros2 topic list

    • ros2 topic info /chatter

Deliverables

  • The two C++ source files for publisher and subscriber.

  • The updated CMakeLists.txt snippet.

  • A short text note:

    • Describe what happens if the subscriber uses topic name "chatter2" instead of "chatter".

    • Describe what happens if you change the message type in the subscriber to geometry_msgs/msg/Twist while leaving the publisher unchanged.

Exercise 5 – Debugging and Introspection Scenario

Goal

Practice using ROS 2 introspection tools to debug a small system.

Scenario

Assume the following:

  • You start your publisher node (from Exercise 4: minimal_publisher).

  • You start your subscriber node (from Exercise 4: minimal_subscriber).

  • The subscriber prints nothing, and you see no log output.

Task

  1. Using only ROS 2 CLI tools (do not modify the code at this stage), list and record the outputs of:

    • ros2 node list

    • ros2 topic list

    • ros2 topic info /chatter

  2. Consider the following possible causes and explain how you would detect each one:

    • The publisher and subscriber are using different topic names.

    • The publisher and subscriber are using different message types.

    • One of the nodes did not start or crashed immediately.

    • You forgot to source the workspace after building.

  3. Propose a systematic checklist (4–6 steps) that you would follow whenever “I see no messages on my subscriber” happens in ROS 2.

Deliverables

  • A short report (about half a page) that includes:

    • The CLI commands you would run and what information you expect.

    • A numbered checklist for debugging publisher–subscriber issues.

    • A final paragraph connecting this process back to the concepts from Lecture 10 (nodes, topics, the ROS 2 daemon, overlays).